A communication system can be seen as a facility that enables communication sessions between two or more entities such as user terminals, base stations and/or other nodes by providing carriers between the various entities involved in the communications path. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communication sessions may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and/or content data and so on. Non-limiting examples of services provided comprise two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.
In a wireless communication system at least a part of a communication session between at least two stations occurs over a wireless link. Examples of wireless systems comprise public land mobile networks (PLMN), satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). The wireless systems can typically be divided into cells, and are therefore often referred to as cellular systems.
A user can access the communication system by means of an appropriate communication device or terminal. A communication device of a user is often referred to as user equipment (UE). A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling communications, for example enabling access to a communication network or communications directly with other users. The communication device may access a carrier provided by a station, for example a base station of a cell, and transmit and/or receive communications on the carrier.
The communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. An example of attempts to solve the problems associated with the increased demands for capacity is an architecture that is known as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. The LTE is being standardized by the 3rd Generation Partnership Project (3GPP). The various development stages of the 3GPP LTE specifications are referred to as releases. Certain releases of 3GPP LTE (e.g., LTE Rel-11, LTE Rel-12, LTE Rel-13) are targeted towards LTE-Advanced (LTE-A). LTE-A is directed towards extending and optimizing the 3GPP LTE radio access technologies.
Communication systems may be configured to use a mechanism for aggregating radio carriers to support wider transmission bandwidth. In LTE this mechanism is referred to as carrier aggregation (CA) and can, according to LTE Rel. 12 specifications, support a transmission bandwidth up to 100 MHz. A communication device with reception and/or transmission capabilities for CA can simultaneously receive and/or transmit on multiple component carriers (CCs) corresponding to multiple serving cells, for which the communication device has acquired/monitors system information needed for initiating connection establishment. When CA is configured, the communication device has only one radio resource control (RRC) connection with the network. At RRC connection establishment/reestablishment or handover, one serving cell provides the non-access stratum (NAS) mobility information, such as tracking area identity information. At RRC connection (re)establishment or handover, one serving cell provides the security input. This cell is referred to as the primary serving cell (PCell), and other cells are referred to as the secondary serving cells (SCells). Depending on capabilities of the communication device, SCells can be configured to form together with the PCell a set of serving cells under CA. In the downlink, the carrier corresponding to the PCell is the downlink primary component carrier (DL PCC), while in the uplink it is the uplink primary component carrier (UL PCC). A SCell needs to be configured by the network using RRC signaling before usage in order to provide necessary information, such as DL radio carrier frequency and physical cell identity (PCI) information, to the communication device. A SCell for which such necessary information has been provided to a communication device is referred to as configured cell for this communication device. The information available at the communication device after cell configuration is in particular sufficient for carrying out cell measurements. A configured SCell is in a deactivated state after cell configuration for energy saving. When a SCell is deactivated, the communication device does in particular not monitor/receive the physical dedicated control channel (PDCCH) or physical downlink shared channel (PDSCH) in the SCell. In other words the communication device cannot communicate in a SCell after cell configuration, and the SCell needs to be activated before data transmission from/the communication device can be initiated in the SCell. LTE provides for a mechanism for activation and deactivation of SCells via media access control (MAC) control elements to the communication device.
Communication systems may be configured to support simultaneous communication with two or more access nodes. In LTE this mechanism is referred to as dual connectivity (DC). More specifically, a communication device may be configured in LTE to communicate with a master eNB (MeNB) and a secondary eNB (SeNB). The MeNB may typically provide access to a macrocell, while the SeNB may provide on a different radio carrier access to a relatively small cell, such as a picocell. Only the MeNB maintains for the communication device in DC mode a connection via an S1-MME interface with the mobility management entity (MME), that is, only the MeNB is involved in mobility management procedures related to a communication device in DC mode. LTE supports two different user plane architectures for communication devices in DC mode. In the first architecture (split bearer) only the MeNB is connected via an S1-U interface to the serving gateway (S-GW) and the user plane data is transferred from the MeNB to the SeNB via an X2 interface. In the second architecture the SeNB is directly connected to the S-GW, and the MeNB is not involved in the transport of user plane data to the SeNB. DC in LTE reuses with respect to the radio interface concepts introduced for CA in LTE. A first group of cells, referred to as master cell group (MCG), can be provided for a communication device by the MeNB and may comprise one PCell and one or more SCells, and a second group of cells, referred to as seconday cell group (SCG), is provided by the SeNB and may comprise a primary SCell (PSCell) with functionality similar to the PCell in the MCG, for example with regard to uplink control signaling from the communication device. This second group of cells may further comprise one or more SCells.
Future networks, such as 5G, may progressively integrate data transmissions of different radio technologies in a communication between one or more access nodes and a communication device. Accordingly, communication devices may be able to operate simultaneously on more than one radio access technology, and carrier aggregation and dual connectivity may not be limited to the use of radio carriers of only one radio access technology. Rather, aggregation of radio carriers according to different radio access technologies and concurrent communication on such aggregated carriers may be supported. Small cells, such as picocells, may progressively be deployed in future radio access networks to match the increasing demand for system capacity due to the growing population of communication devices and data applications. Integration of radio access technologies and/or a high number of small cells may bring about that a communication device may detect more and more cells in future networks which are suitable candidates for connection establishment. Enhancements of carrier aggregation and dual connectivity mechanisms may be needed to make best use of these cells in future radio access networks. Such enhancements may allow for an aggregation of a high number of radio carriers at a communication device, for example up to 32 are currently specified in LTE Rel. 13, and in particular an integration of radio carriers operated on unlicensed spectrum.
Aggregation of radio carriers for communication to/from a communication device and simultaneous communication with two or more access nodes may in particular be used for operating cells on unlicensed (license exempt) spectrum. Wireless communication systems may be licensed to operate in particular spectrum bands. A technology, for example LTE, may operate, in addition to a licensed band, in an unlicensed band. LTE operation in the unlicensed spectrum may be based on the LTE Carrier Aggregation (CA) framework where one or more low power secondary cells (SCells) operate in the unlicensed spectrum and may be either downlink-only or contain both uplink (UL) and downlink (DL), and where the primary cell (PCell) operates in the licensed spectrum and can be either LTE Frequency Division Duplex (FDD) or LTE Time Division Duplex (TDD).
Two proposals for operating in unlicensed spectrum are LTE Licensed-Assisted Access (LAA) and LTE in Unlicensed Spectrum (LTE-U). LTE-LAA specified in 3GPP as part of Rel. 13 and LTE-U as defined by the LTE-U Forum may imply that a connection to a licensed band is maintained while using the unlicensed band. Moreover, the licensed and unlicensed bands may be operated together using, e.g., carrier aggregation or dual connectivity. For example, carrier aggregation between a primary cell (PCell) on a licensed band and one or more secondary cells (SCells) on unlicensed band may be applied, and uplink control information of the SCells is communicated in the PCell on licensed spectrum.
In an alternative proposal stand-alone operation using unlicensed carrier only may be used.
In standalone operation at least some of the functions for access to cells on unlicensed spectrum and data transmission in these cells are performed without or with only minimum assistance or signaling support from license-based spectrum. Dual connectivity operation for unlicensed bands can be seen as an example of the scenario with minimum assistance or signaling from licensed-based spectrum.
Unlicensed band technologies may need to abide by certain rules, e.g. a clear channel assessment procedure, such as Listen-Before-Talk (LBT), in order to provide fair coexistence between LTE and other technologies such as Wi-Fi as well as between LTE operators. In some jurisdictions respective rules may be specified in regulations.
In LTE-LAA, before being permitted to transmit, a user or an access point (such as eNodeB) may, depending on rules or regulatory requirements, need to perform a Clear Channel Assessment (CCA) procedure, such a Listen-Before-Talk (LBT). The user or access node may, for example, monitor a given radio frequency, i.e. carrier, for a short period of time to ensure that the spectrum is not already occupied by some other transmission. The requirements for CCA procedures, such as LBT, vary depending on the geographic region: e.g. in the US such requirements do not exist, whereas in e.g. Europe and Japan the network elements operating on unlicensed bands need to comply with LBT requirements. Moreover, CCA procedures, such as LBT, may be needed in order to guarantee co-existence with other unlicensed band usage in order to enable e.g. fair co-existence with Wi-Fi also operating on the same spectrum and/or carriers. After a successful CCA procedure the user or access point is allowed to start transmission within a transmission opportunity.
The maximum duration of the transmission opportunity may be preconfigured or may be signaled in the system, and may extend over a range of 4 to 13 milliseconds. The access node may be allowed to schedule downlink (DL) transmissions from the access node and uplink (UL) transmissions to the access node within a certain time window. An uplink transmission may not be subject to a CCA procedure, such as LBT, if the time between a DL transmission and a subsequent UL transmission is less than or equal to a predetermined value. Moreover, certain signaling rules, such as Short Control Signaling (SCS) rules defined for Europe by ETSI, may allow for the transmission of control or management information without LBT operation, if the duty cycle of the related signaling does not exceed a certain threshold, e.g. 5%, within a specified period of time, for example 50 ms. The aforementioned SCS rules, for example, can be used by compliant communication devices, referred to as operating in adaptive mode for respective SCS transmission of management and control frames without sensing the channel for the presence of other signals. The term “adaptive mode” is defined in ETSI as a mechanism by which equipment can adapt to its environment by identifying other transmissions present in a band, and addresses a general requirement for efficient operation of communications systems on unlicensed bands. Further, scheduled UL transmissions may in general be allowed without LBT, if the time between a DL transmission from an access node and a subsequent UL transmission is less than or equal to a predetermined value, and the access node has performed a clear channel assessment procedure, such as LBT, prior to the DL transmission. The total transmission time covering both DL transmission and subsequent UL transmission may be limited to a maximum burst or channel occupancy time. The maximum burst or occupancy time may be specified, for example, by a regulator.
Data transmission on an unlicensed band or/and subject to a clear channel assessment procedure cannot occur pursuant to a predetermined schedule in a communication system. Rather, communication devices and access nodes need to determine suitable time windows for uplink transmission and/or downlink transmission. A respective time window may comprise one or more transmission time intervals (TTI), such as subframes in LTE, and is in the following referred to as uplink transmission opportunity or downlink transmission opportunity. A TTI is the time period reserved in a scheduling algorithm for performing a data transmission of a dedicated data unit in the communication system. The determination of uplink transmission opportunities and/or downlink transmission opportunities may be based on parameters related to the communication system, such as a configured pattern governing the sequence of uplink and downlink transmissions in the system. The determination may further be based on rules or regulations specifying a minimum and/or maximum allowed length of uplink transmissions and/or downlink transmissions. The determination of uplink and downlink opportunities may in particular be based on the outcome of a clear channel assessment procedure, and communication devices or access nodes will only start data transmission on a frequency band after having assessed that the frequency band is clear, that is, not occupied by data transmissions from other communication devices or access nodes. Further rules or regulations may govern data transmissions in a communication between an access node and one or more communication devices. These rules may, for example, specify a maximum length of a time window in the communication covering at least one transmission in a first direction, for example in DL in a cellular system from an access node of a cell, and at least one subsequent transmission in the reverse direction, for example in UL from one or more communication devices in the cell. Such a time window comprising one or more DL and UL transmissions is in the following referred to as communication opportunity. DL transmissions may comprise scheduling information which may be transmitted on a DL control channel. The scheduling information may in particular be used for scheduling one or more UL data transmissions and/or one or more DL data transmissions within the current one or more future communication opportunities.
Scheduling information for a data transmission is indicative of an assignment of contents attributes, format attributes and mapping attributes to the data transmission. Mapping attributes relate to one or more channel elements allocated to the transmission on the physical layer. Specifics of the channel elements depend on the radio access technology and may depend on the used channel type. A channel element may relate to a group of resource elements, while each resource element relates to a frequency attribute, for example a subcarrier index (and the respective frequency range) in a system employing orthogonal frequency-division multiplexing (OFDM), and a time attribute, such as the transmission time of an OFDM or Single-Carrier FDMA symbol. A channel element may further relate to a code attribute, such as a cover code or a spreading code, which may allow for parallel data transmission on the same set of resource elements. Illustrative examples for channel elements in LTE are control channel elements (CCE) on the physical downlink control channel (PDCCH) or the enhanced physical downlink control channel (EPDCCH), PUCCH resources on the physical uplink control channel (PUCCH), and physical resource blocks (PRB) on the physical downlink shared channel (PDSCH) and the physical uplink shared channel (PUSCH). It should be understood that each data transmission is associated with the code attributes of the allocated channel elements and the frequency and time attributes of the resource elements in the allocated channel elements. Format attributes relate to the processing of a set of information bits in the transmission prior to the mapping to the allocated channel elements. Format attributes may in particular comprise a modulation and coding scheme used in the transmission and the length of the transport block in the transmission. Contents attributes relate to the user/payload information conveyed through the transmission. In other words, a contents attribute is any information which may in an application finally affect the arrangement of a detected data sequence at the receiving end. Contents attributes may comprise the sender and/or the receiver of the transmission. Contents attributes may further relate to the information bits processed in the transmission, for example some kind of sequence number in a communication. Contents attributes may in particular indicate whether the transmission is a retransmission or relates to a new set of information bits. In case of a hybrid automatic repeat request (HARQ) scheme contents attributes may in particular comprise an indication of the HARQ process number, that is, a HARQ-specific sequence number, the redundancy version (RV) used in the transmission and a new data indicator (NDI).
Scheduling information for a data transmission need not comprise assignment information for the complete set of attributes needed in the data transmission. At least a part of the attributes can be preconfigured, for example through semi-persistent scheduling, and can be used in more than one data transmission. Some of the attributes may be signaled implicitly or may be derivable, for example from timing information. However, dynamic scheduling in a more complex system, such as a cellular mobile network, requires transmission of scheduling information on a DL control channel. In a system employing carrier aggregation the DL scheduling information related to a certain data transmission may be transmitted on a component carrier other than the data transmission. Transmission of a data and scheduling information on different component carriers is referred to as cross-carrier scheduling.
In a cell operated on unlicensed spectrum a communication device may start monitoring channel elements related to a DL control channel carrying scheduling information after detection of DL data burst in the cell. The detection of the DL data burst may be based on the detection of a certain signal in the cell, for example a reference signal, such as a cell reference signal which the communication device may blindly detect, or based on explicit signaling indicative of the presence of the DL data burst. Monitoring channel elements related to a DL control channel may comprise blind detection of scheduling information destined to the communication device. The control channel may be a physical downlink control channel (PDCCH) or enhanced physical downlink control channel (EPDCCH) as specified in LTE or a similar channel. The communication device may further detect a DL data transmission on a data channel, such as a physical downlink shared channel (PDSCH) or a similar channel, based on the detected scheduling information.
A communication system may employ a retransmission mechanism, such as Automatic Repeat Request (ARQ), for handling transmission errors. A receiver in such a system may use an error-detection code, such as a Cyclic Redundancy Check (CRC), to verify whether a data packet was received in error. The receiver may notify the transmitter on a feedback channel of the outcome of the verification by sending an acknowledgement (ACK) if the data packet was correctly received or a non-acknowledgement (NACK) if an error was detected. The transmitter may subsequently transmit a new data packet related to other information bits, in case of an ACK, or retransmit the data packet received in error, in case of a NACK. The retransmission mechanism may be combined with forward error-correction coding (FEC), in which redundancy information is included in the data packet prior to transmission. This redundancy information can be used at the receiver for correcting at least some of the transmission errors, and retransmission of a data packet is only requested in case of uncorrectable errors. Such a combination of FEC and ARQ is referred to as hybrid automatic repeat request (HARQ). In a HARQ scheme the receiver may not simply discard a data packet with uncorrectable errors, but may combine obtained information with information from one or more retransmissions related to the same information bits. These retransmissions may contain identical copies of the first transmission. In more advanced schemes, such as incremental redundancy (IR) HARQ, the first transmission and related retransmissions are not identical. Rather, the various transmissions related to the same information bits may comprise different redundancy versions (RV), and each retransmission makes additional redundancy information available at the receiver for data detection. The number of transmissions related to the same information bits may be limited in a communication system by a maximum number of not successful transmissions, and a data packet related to new information bits may be transmitted once the maximum number of not successful transmissions has been reached. A scheduling grant may comprise a new data indicator (NDI) notifying a communication device whether the scheduled transmission is destined for a data packet related to new information bits. Further or alternatively, the scheduling grant may comprise an indication of the redundancy version (RV) used or to be used in the transmission. Each data packet, often referred to as transport block, may be transmitted in a communication system within a transmission time interval (TTI), such as a subframe in LTE. At least two transport blocks may be transmitted in parallel in a TTI when spatial multiplexing is employed. Processing of a transport block, its transmission and the processing and transmission of the corresponding HARQ-ACK feedback may take several TTIs. For example, in LTE-FDD such a complete HARQ loop takes eight subframes. Accordingly, eight HARQ processes are needed in a data stream in LTE-FDD for continuous transmission between an access node and a communication device. The HARQ processes are handled in the access nodes and the communication devices in parallel, and each HARQ process controls the transmission of transport blocks and ACK/NACK feedback related to a set of information bits in the data stream.
The following relates to UL HARQ. Specifically, it relates to scheduling schemes for UL data transmission in a TDD system.
In a conventional LTE-TDD system a communication device expects HARQ-ACK feedback transmission in DL to occur according to a predefined timing in relation to the subframe (TTI) in which a transport block was transmitted in UL on a physical uplink shared channel (PUSCH). Specifically, a communication device expects HARQ-ACK feedback for an UL data transmission in subframe n to be provided in subframe n+k_PHICH either on the physical hybrid ARQ indicator channel (PHICH) or by receiving a new UL grant on a physical downlink control channel (PDCCH) or enhanced physical downlink control channel (EPDCCH). HARQ-ACK feedback transmission on PHICH causes non-adaptive retransmission, that is, transmission on the same UL resource elements, while transmission on PDCCH allows for adaptive retransmission, that is, UL transmission according to new/adapted scheduling information. The HARQ-ACK delay k_PHICH depends in LTE-TDD on the selected UL/DL configuration as well as the subframe number n of the UL data transmission on PUSCH. Table 1 shows the association between UL/DL configurations, subframe number n and the corresponding HARQ-ACK delay k_PHICH as specified in 3GPP specification TS 36.213. The UL/DL configurations 0-6 in Table 2 specify the allocation of subframes to UL and DL in a radio frame and the positions of special subframes S in a radio frame which allow for a switching from one transmit direction to the other.
TABLE 1k_PHICH for LTE-TDDTDD UL/DLsubframe number nConfiguration0123456789047647614646266366646656646647
TABLE 2Uplink-downlink configurations in LTE-TDDDownlink-to-UplinkUplink-Switch-downlinkpointSubframe numberconfigurationperiodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The minimum HARQ-ACK delay k_PHICH in Table 1 is four subframes and corresponds to the HARQ processing delay k_PROC. The HARQ processing delay k_PROC includes internal processing times of a communication device and transmission delays to and from the communication device (including also timing advance), and extends at the access node from the beginning of the subframe/TTI containing HARQ-ACK feedback to the beginning of the subframe/TTI in which the access node receives the corresponding UL transmission. Additional delays k_PHICH>k_PROC occur if the communication device has to wait for the next uplink subframe U for starting the transmission.
UL/DL configurations in LTE-TDD can be construed as communication opportunities with predetermined/predictable occurrence and length. As discussed above such a predetermined/predictable activity pattern cannot be ensured in a cell operating on unlicensed spectrum due to the uncoordinated access from different network operators and/or radio access networks. Therefore, it has been proposed to use an asynchronous transmission scheme for UL HARQ on unlicensed spectrum. In such a scheme UL retransmissions can be scheduled by UL grants and can occur without predetermined offsets in relation to the subframe of the initial transmission. However, even such an asynchronous transmission scheme has to consider the HARQ processing delay k_PROC between a detected HARQ-ACK feedback and the corresponding UL data transmission. This limits the ability of the scheme to adapt UL and DL transmissions in a communication in response to channel occupancy problems.
Therefore, there is a need to provide a signaling scheme for scheduling decisions which allows for dynamic adaption of UL and DL transmission sequences in a communication under consideration of HARQ processing requirements.